A Sand Glass, also known as an hour glass or sandglass, sand timer or sand clock, is a device for the measurement of time. It consists of two glass bulbs placed one above the other which are connected by a narrow tube. One of the bulbs is usually filled with fine sand which flows through the narrow tube into the bottom bulb at a given rate. Once all the sand has run to the bottom bulb, the device can be inverted in order to measure time again. The hourglass is named for the most frequently used sandglass, where the sands have a running time of one hour.

Hourglasses are said to have been invented at Alexandria about the middle of the third century, where they were sometimes carried around as people carry watches today. It is speculated that it was in use in the 11th century, where it would have complemented the magnetic compass as an aid to navigation.

Early sand glasses, such as this one, consisted of a pair of glass bulbs with their necks in contact with a metal plate into which a small hole had been drilled, and which were bound together by thread. By the middle of the 18th century they were blown in one piece, with a small hole through which the sand could be inserted. The time taken for one bulb of the sand glass to empty is a definite interval, at the end of which the glass can be turned over and the process repeated. Sand glasses are useful for measuring long periods of time, but prove inadequate for short time intervals.

Glassmaking was brought to Europe in the thirteenth century by the Venetians, who created notable sandglasses. Recorded evidence of their existence is found no earlier than the 14th century, the earliest being an hourglass appearing in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti. Written records from the same period mention the hourglass, and it appears in lists of ships stores. One of the earliest surviving records is a sales receipt of Thomas de Stetesham, clerk of the English ship La George, in 1345:

The same Thomas accounts to have paid at Lescluse, in Flanders, for twelve glass horologes (" pro xii. orlogiis vitreis "), price of each 4 gross', in sterling 9s. Item, For four horologes of the same sort (" de eadem secta "), bought there, price of each five gross', making in sterling 3s. 4d.

On board a ship in the age of sail, it would be responsibility of the navigator or ship's boy to flip the sand glass as soon as it ran out and to subsequently ring the bell to indicate the amount of time that had passed. Skipping or missing a flip would have profound impacts on work schedules, navigational calculations, and more and would generally result in harsh discipline.

The size of an hourglass is not necessarily the deciding factor for its running time. However, if its running time is to amount to several days or weeks, it will need to be fairly large. Two such giants include the Time Wheel in Budapest and the hourglass at the sand museum in the Japanese city of Nima. At eight and six metres in height respectively and a running time of one year, they are among the world's largest chronometers. Another giant has been standing at the Red Square in Moscow since July 2008. At 11.90 m in height and weighing 40 tonnes, this is likely the largest hourglass in the world. Both cones of the hourglass are large enough that a 5 m long BMW can easily be accommodated inside. By way of comparison the smallest hourglass in the world is just 2.4 cm high. It was made in 1992 in Hamburg and takes slightly less than 5 seconds for a single run through.

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